The present invention relates to the preparation of hexamethylcyclotrisilazane from octamethylcyclotetrasilazane. Hexamethylcyclotrisilazane is useful as a difunctional blocking agent which is employed to derivatize and to protect various substrates during chemical analysis and synthetic reactions.
Silane blocking agents are used to replace active hydrogens with silyl groups. The silyl derivatives of various alcohols, carboxylic acids, phenols, amino acids, carbohydrates and amines may be more volatile than the corresponding active hydrogen containing substrates and consequently more easily analyzable by gas chromatography. In addition, silane blocking agents are very useful synthetic intermediates, because replacement of active hydrogens with silyl groups often affords products which are more chemically stable and which undergo subsequent chemical reaction at sites other than the silyl blocked site. Hydrolysis of the silyl blocked site will subsequently regenerate the unprotected active hydrogen functionality.
The most commonly used monofunctional silane blocking agent used for silylation is hexamethyldisilazane. This compound reacts with protic species such as alcohols, phenols, carboxylic acids, amino acids, carbohydrates, thiols and amines yielding the silylated derivatives and ammonia as a by-product. Difunctional silane blocking agents are available to selectively block primary amines, vic-diols and other difunctional sites, typically yielding 5-, 6-, and 7-membered silicon-containing rings which are very stable to hydrolysis, oxidation and reduction. This process is termed cyclosilylation.
Examples of effective difunctional silane blocking agents for rigid molecules such as steroids, salicylic, thiosalicylic and anthranilic acids are dimethyldiacetoxysilane and dimethyldimethoxysilane. A more broadly useful technique for cyclosilylation is reaction with hexamethylcyclotrisilazane which forms stable blocked derivatives without formation of polymeric by-products and without need for bulky groups to be present on the substrate compound as discussed in L. Birkofer and O. Stuhl, J. Organomet. Chem., 177, C16 (1979); Ibid. 164, C16 (1979).
Hexamethylcyclotrisilazane may be prepared by the direct ammonolysis of dimethyldichlorosilane. S. D. Brewer, C. P. Haver, J. Am. Chem. Soc., 70, 3888 (1948). However, direct ammonolysis yields a mixture of hexamethylcyclotrisilazane and octamethylcyclotetrasilazane. Hexamethylcyclotrisilazane may be separated from the reaction mixture by fractionation. It is desirable to maximize the proportion of cyclic trimer obtained per unit of raw materials.
Other cyclic silazanes may be prepared by direct ammonolysis of substituted dihalosilanes. For example, U.S. Pat. No. 2,885,370 discloses that cyclic trisilazanes may be prepared by reaction of dicarbocylic dihalosilane such as diphenyldichlorosilane, dihexyldichlorosilane, di-p-tolyldichlorosilane, diphenyldibromosilane, and the like, with ammonia, either in anhydrous ammonia, or in an inert anhydrous liquid medium. Hexaethylcyclotrisilazane and octaethylcyclotetrasilazane are also reported by Brewer et al. U.S. Pat. No. 3,481,964 discloses the preparation of cyclotetrasilazane and cyclotrisilazanes by an analogous process, wherein the dihalogen silane may be substituted with aryloxy, alkenyl including vinyl, aryloxy et al. Octamethylcyclotetrasilazane may be prepared by heating hexamethylcyclotrisilane and hexamethyldisilazane with ammonium chloride. Brewer et al. The oligomerization and polymerization of hexamethylcyclotrisilazane is reviewed in E. G. Rochow, IUPAC Organosilicon Chemistry (1966) pp. 247-262.
Recently the catalytic activation of the Si-N bond as a means of producing high molecular weight oligomers containing the --(CH.sub.3).sub.2 SiNH-- unit has been described. M. T. Zoeckler, R. M. Laine, J. Org. Chem., 48, 2539 (1983); R. M. Laine, Y. Blum, A.C.S. Spring Meeting Preprints (May 1, 1985). Using ruthenium carbonyl, Ru.sub.3 (CO).sub.12, as a catalyst to promote activation of Si-N bonds, linear oligomers have been prepared from mixtures of octamethyltetrasilazane and hexamethyldisilazane. Polymers prepared by this method may be pyrolytically converted to yield silicon nitride and silicon carbide nitride. D. Seyferth and G. H. Wiseman, Prepr. of the Polymer Chemistry Div. A.C.S. 10 (1984).